1. Field of the Invention
This invention relates to a method for coating implantable medical devices, such as stents.
2. Description of the Background
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially press against the atherosclerotic plaque of the lesion for remodeling of the vessel wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Vasospasms and recoil of the vessel wall also threaten vessel closure. Moreover, thrombosis and restenosis of the artery can develop over several months after the procedure, which can require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents can be used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents can act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically stents are capable of being compressed, so that they can be inserted through small lumens via catheters, and then expanded to a larger diameter once they are at the desired location.
Although stents work well mechanically, the chronic issues of restenosis and, to a lesser extent, thrombosis remain. These events are adversely affected by the mechanical aspects of the stent such as the degree of injury and disturbance in hemodynamics caused by the stent. To the extent that the mechanical functionality of stents has been optimized, it has been postulated that continued improvements could be made by pharmacological therapies. Many systemic therapies have been tried. A challenge is maintaining the necessary concentration of a therapeutic substance at the lesion site for the necessary period of time. This can be done via brute force methods using oral or intravenous administration but the issues of systemic toxicity and side effects arise. Therefore, a preferred route can be achieved by local delivery of a therapeutic substance from the stent itself. Being made of metal, plain stents are not useful for therapeutic substance delivery. Therefore, a coating, usually made from a polymer, is applied to serve as a therapeutic substance reservoir. A solution of a polymer dissolved in a solvent and a therapeutic substance added thereto is applied to the stent and the solvent is allowed to evaporate. Accordingly, a polymeric coating impregnated with a therapeutic substance remains on the surface of the stent. The polymeric coating can include multiple layers. A primer composition, free from any drugs, can be applied on the surface of the device. A polymer solution including the drug can then be applied on the primer layer. To reduce the rate of release of the drug, a topcoat layer can be applied over the reservoir layer. The application of each layer can be performed subsequent to the drying of the previous layer.
In order to be effectively applied with conventional spraying or dipping techniques, the coating solution needs to have a low viscosity. Low viscosities can be achieved by adding a higher fraction of solvent to the solution or by changing the composition of the solution with the addition of a “wetting fluid.” Compositions having a low viscosity require multiple applications of the composition and evaporation of the solvent in order to obtain a coating of suitable thickness or weight, as compared to using compositions having greater viscosities. Accordingly, it is desired to use more viscous compositions to reduce the number of application steps and in effect reduce the processing time of forming the coating.
The topcoat layer is intended to reduce the rate of release of the drug from the polymer-drug layer. However, when a topcoat composition is applied to the polymer-drug layer, the solvent may extract the drug out from the polymer-drug layer, therefore reducing the effectiveness of the topcoat layer. Accordingly, it is desired to prevent the extraction of the drug out from the drug-polymer layer when a top coat layer is applied.